1. Field of the Invention:
This invention relates to a process for deinking printed waste paper by treatment of the paper pulp in a pulper at alkaline pH values using alkali metal silicate, oxidative bleaches, higher fatty acids and/or resinic acids and dispersants and separation of the detached printing ink particles from the fiber suspension by flotation.
2. Statement of Related Art:
Deinking comprises the selective removal of printing inks from waste paper fiber suspensions. The reusable material thus obtained is processed to end products, such as for example writing paper, printing paper, toilet paper, etc., depending on the particular starting material.
Important steps in the deinking process are, first, the removal of the printing inks from the fibers in a pulper using suitable chemicals and, subsequently, their selective removal from the fiber suspension. Both in flotation deinking and in washing deinking, these steps are carried out in alkaline medium, the printing inks being removed in flotation cells or washers. The reuseable material obtained is then processed into the desired products.
A typically used chemical formulation may have the following composition (quantities based on the dry matter content of the waste paper):
0.5 to 2.0% by weight alkali metal hydroxide (100%) PA0 0.2 to 1.5% by weight soap or fatty acid (100%) PA0 0.05 to 0.1% by weight nonionic dispersant (100%) PA0 2.0 to 5.0% by weight alkali metal silicate (waterglass) PA0 0.1 to 0.3% by weight diethylenetriamine pentacetic acid, Na salt (DTPA) (40%) PA0 0.5 to 3.0% by weight oxidative bleach (100%). PA0 (a) treatment of the paper pulp in a pulper at alkaline pH values using alkali metal silicate, oxidative bleaches, higher fatty acids and/or resinic acids, dispersants and PA0 (b) separation of the detached printing ink particles from the fiber suspension by flotation, PA0 (a) 0.5 to 2% by weight sodium hydroxide (100%); PA0 (b) 2 to 5% by weight alkali metal silicate; PA0 (c) 0.1 to 0.3% by weight diethylenetriamine pentaacetic acid, sodium salt (DTPA) (40% aqueous solution); PA0 (d) 0.5 to 3% by weight of an oxidative bleach, e.g. hydrogen peroxide (100%); PA0 (e) 0.2 to 1.5% by weight oil-in-water dispersion of fatty acid and/or resinic acid calcium salts; PA0 (f) 0 to 0.5% by weight nonionic dispersants,
The mechanisms by which the individual chemical act in the flotation process are as follows:
Alkali metal hydroxide, preferably sodium hydroxide, softens the fibers and hence facilitates breaking up of the inter-fiber bonds and removal of the printing ink. Removal of the printing ink is additionally promoted by neutralization or saponification of printing ink binders. In addition, sodium hydroxide provides the alkalinity required to neutralize fatty acids which is essential to their function as collectors.
Nonionic dispersants improve wetting of the fibers by lowering the surface tension of the water, which promotes, inter alia, the effect of the other chemicals, and is largely responsible for the detachment of the printing inks and their dispersion. The dispersion of the printing inks prevents them from becoming, or makes it difficult for them to become, reattached to the fibers.
The detached pigments are hydrophobicized by the collectors, i.e. fatty acids or soaps, so that selective removal is possible. In addition to the alkaline medium, the salts responsible for the hardness of water are crucially important to this function where known anionic collectors are used, because it is only the calcium salts which hydrophobicize the printing ink particles and thus make them floatable. Oxidative bleaches prevent or compensate for the yellowing of the wood-containing waste paper and, in addition, also bleach the fibers, depending on the dosage used. The bleaches are stabilized by alkali metal silicate and DTPA which, by complexing the heavy metals, prevent overrapid decomposition of the bleach.
The waste paper used, depending on the desired end product, consists of newspapers, magazines, computer papers, files, etc., but in most cases mixtures thereof, deinkability being crucially determined by the printing ink systems used and their age.
Evaluation of the paper after the deinking process is based on whiteness measurement, the whiteness of the test sheet being compared with that of a standard (for example barium sulfate where whiteness is measured by the Elrepho system) and expressed in percent. For example, a waste paper moisture of daily newspapers and magazines in a ratio of 1:1 gives a whiteness of 56 to 60%, the whiteness of the unprinted margins of these waste papers being around 65 to 68%.
Published German application (DE-OS) No. 29 03 150 describes a process for deinking printed waste paper by treatment of the paper pulp in a pulper at alkaline pH values using alkali silicate, oxidative bleaches, higher fatty acids or salts thereof, and nonionic dispersants, with separation of the detached printing ink particles from the fiber suspension by flotation, a fatty acid alkanolamide additionally being used in the pulper.
By contrast, German Patent No. 31 23 353 describes a similar process which differs from the process disclosed in DE-OS No. 29 03 150 firstly in that no fatty acid alkanolamide is used in the pulper and secondly in that the fatty acids and/or resinic acids and the dispersants are added in the form of an oil-in-water emulsion.
However, these known processes are attended by certain disadvantages.
Where solid fatty acids are used as collectors, melting apparatus or heated storage vessels are required where the fatty acids are supplied in molten form.
There is no need for heating where the fatty acids used melt at temperatures below the normal temperature range, although fatty acids such as these are generally attended by disadvantages in terms of practical application.
Under the reaction conditions prevailing in the pulper, the relatively high melting and liquid fatty acids saponify only slowly, the soaps formed at the surface of the fatty acid particles complicating or preventing further saponification of the enclosed fatty acids. Not only does this result in a reduction in the collector effect, it can also lead to intensified fiber flotation because the fibers are hydrophobicized by the unsaponified fatty acids.
To overcome these disadvantages, the storage tanks used in the prior art are followed by saponification units. The resulting aqueous soap solutions thus have to be permanently "tempered" pending processing because they form gels at normal temperature, i.e. the intermediate storage vessels and also the metering pipes, including the metering head of the metering pumps, have to be heat-insulated and fully heated.
Where solid soaps are used, there is no need for a saponification unit. Since, in most cases, the soap granulates are not completely soluble under the process-dependent conditions, dissolving units have to be installed to prevent negative effects. The resulting soap solutions are then attended by the same disadvantages as described above.
Another significant disadvantage attending known processes lies in the fact that the above-discussed formation of calcium soaps of the fatty acids, which of course act as collectors, only takes place during the deinking process in the pulper with the hardness salts of the water used. The reaction time required for this purpose thus extends the overall process time. In addition, an incomplete reaction gives poorer deinking results which are reflected, for example, in reduced whiteness levels. These disadvantages also attend processes in which the fatty acids are used in the form of an emulsion.